@ v0dka:
take
(x,y)=(1.35,2650)
then, for 1.5V you should be able to achieve 2770MHz, according to the formula. That corresponds to the range you determined (2700-2800@1.5V)! It is not possible to exclude the possibility of a linear relation with your two single points.

Did you try out the table that you posted? Do the values match with your findings?